Laser marking, also called laser engraving, refers to using a laser to make a readable mark on an object. Unlike traditional marking or engraving techniques, laser marking does not involve the use of inks or tool bits which come in contact with the target surface and need to be regularly replaced. Rather, with laser marking, a laser is used to remove portions of the target material to produce permanent marks. Specifically, the laser power is absorbed by the target material where the laser touches its surface, causing a rapid increase in temperature that vaporizes a portion of the target material, leaving a permanent mark. Laser marking is particularly useful in production, product distribution, and quality control applications.
Typically, high average power lasers with an average power of greater than 10 W or high pulse energy lasers with pulse energy near 1 mJ are used for laser marking applications. Examples of lasers that are commonly used include CO2 lasers at 10.6 micron wavelength, ND; YAG lasers at 1064 nm, frequency doubled and tripled 532 nm and 355 nm lasers, and Yb-doped fiber lasers near 1 μm. Normally, the laser and target material are matched such that the target material exhibits a strong absorption at the laser wavelength being used. When the power and energy are increased even further, the laser can be used to cut or drill holes on the target material.
As polymers are widely used for industrial and consumer applications, the ability to efficiently laser mark polymer materials is important. For pigmented polymers the process is relatively straight forward as a laser that matches the absorption wavelength of the colored polymer material can be used. However, currently the ability to laser mark visually transparent polymers with minimal damage to the target object is limited. The most popular technique is to add pigment into the polymer and to use a UV laser for marking. Often the additive is titanium dioxide and when the laser is directed at the additive-containing polymer, the photosensitive titanium dioxide changes color as a result of the laser-induced reduction of Ti4+ (colorless) to Ti3+ (blue-black) in the titanium dioxide lattice. The use of titanium dioxide in a fluoropolymer is disclosed in U.S. Pat. Nos. 5,560,845 and 5,789,466. Many other types of additives that can be used are disclosed in other U.S. patents, such as U.S. Pat. No. 6,825,265.
However, the requirement to add pigments to transparent polymers in order to utilize laser marking limits its application and increases the complexity of the laser marking process, thereby increasing the overall cost. Thus, there is a need for the ability to laser mark transparent polymers without the use of additives.